Lean-burn engines are operated primarily with a lean air/fuel mixture, i.e., the air/fuel mixture contains more oxygen than is necessary for complete combustion of the fuel. The composition of the air/fuel mixture is frequently described by the air/fuel ratio λ normalized to stoichiometric conditions, hereinafter also referred to as the excess air coefficient. The air/fuel ratio for stoichiometric conditions has a value of 14.7 for conventional engine fuels. The excess air coefficient in this case is 1.0. In the case of air deficiency, i.e., in a rich air/fuel mixture, λ is less than 1.0; in the case of air excess, i.e., in a lean air/fuel mixture, λ is greater than 1.0. In the absence of selective absorption in the engine of specific components of the combustion gases, the exhaust gas leaving the engine has the same excess air coefficient as the air/fuel mixture supplied to the engine.
Lean-burn engines are distinguished by lower fuel consumption as compared to conventional engines, which are operated primarily with a stoichiometric air/fuel mixture. Lean-burn engines include gasoline engines developed for lean-burn operation and diesel engines.
Because of the high oxygen content of the exhaust of the lean-burn engine, it is difficult to reduce the nitrogen oxides (NOx) emitted by the lean-burn engine to nitrogen to make them harmless. To remove nitrogen oxides from the lean exhaust gas of internal combustion engines, so-called nitrogen oxides storage catalysts—hereinafter referred to as storage catalysts for short—were developed. Storage catalysts adsorb the nitrogen oxides in the lean exhaust gas in the form of nitrates and release them again in rich exhaust gas.
The mode of operation and composition of nitrogen oxides storage catalysts is described, for example, in European Patent EP 0 560 991 B1. The storage material used in these catalysts contains at least one component selected from the group consisting of the alkali metals (potassium, sodium, lithium, cesium), the alkaline-earth metals (barium, calcium) or the rare-earth metals (lanthanum, yttrium). The storage catalyst contains platinum as the catalytically active element. Under oxidizing exhaust gas conditions, i.e., in lean-burn operation, the storage materials can store the nitrogen oxides contained in the exhaust gas in the form of nitrates. This requires, however, that the nitrogen oxides, which depending on the engine type and its mode of operation consist of approximately 60 to 95% nitrogen monoxide, are first oxidized to nitrogen dioxide. This occurs on the platinum component of the storage catalyst.
Since the capacity of the storage catalyst is limited, the catalyst must be regenerated from time to time. For this purpose, the excess air coefficient of the air/fuel mixture supplied to the engine, and thus also the excess air coefficient of the exhaust gas leaving the engine, must be lowered briefly to values of less than 1. This is referred to as enriching the air/fuel mixture of the exhaust gas. Thus, during this brief operating phase, reducing conditions are present in the exhaust gas before the exhaust gas enters the storage catalyst.
Under the reducing conditions present during the enrichment phase, the nitrogen oxides stored in the form of nitrates are released again (desorbed) and are reduced to nitrogen on the storage catalyst with simultaneous oxidation of carbon monoxide, hydrocarbons and hydrogen, as in conventional three-way catalysts. This process is hereinafter referred to as NOx regeneration.
The storage phase typically lasts 60 seconds, while NOx regeneration requires about 5 to 20 seconds.
The described process was developed for gasoline engines operated under lean-burn conditions, so-called lean-burn engines. Until a few years ago, this process could be used only to a limited extent in diesel engines because it involved a loss of comfort in the running properties of the diesel engine during the rich phase (German Laid Open Publication DE 196 36 790 A1). Meanwhile, however, engine control systems (e.g., German Patent Specification DE 197 50 226 C1 corresponding to U.S. Pat. No. 6,082,325) that allow brief rich-burn operation of a diesel engine without any noticeable loss of comfort have been developed. As a result, this process can now also be used in diesel engines.
Despite their great potential for removing nitrogen oxides from the exhaust gas of diesel engines, nitrogen oxide storage catalysts are not widely used today. A significant problem encountered in the use of nitrogen oxide storage catalysts is the sulfur content of the fuels, particularly of diesel fuels. Various sulfur compounds are created during combustion—sulfur oxides (SOx) in lean-burn operation. The sulfur oxides poison the storage components of the storage catalyst. This poisoning essentially occurs in the same manner as the storage of the nitrogen oxides. Sulfur dioxide emitted by the diesel engine is oxidized to sulfur trioxide on the catalytically active noble metal component of the storage catalyst. Sulfur trioxide reacts with the storage materials of the storage catalyst in the presence of the water vapor contained in the exhaust gas to form the corresponding sulfates. A particular drawback is that the absorption of sulfur trioxide is preferred compared to the absorption of nitrogen oxides, and the sulfates formed are thermally very stable. Thus, the formation of the sulfates competes with the storage of the nitrogen oxides (NOx), and the nitrogen oxide storage capacity of the catalyst is clearly reduced because of the poisoning with sulfur oxides. Consequently, the stored sulfur oxides must therefore be removed from the storage catalyst from time to time to restore the full nitrogen oxide storage capacity.
Removal of sulfur oxides requires special conditions that are distinct from the conditions for NOx regeneration; in other words, during NOx regeneration there is no removal of sulfur components from the storage catalyst, a process hereinafter referred to as desulfation. For desulfation, the storage catalyst must be operated under reducing conditions, i.e., with rich exhaust. In addition, high exhaust gas temperatures are required to heat the catalyst. The catalyst temperature required for desulfation depends on the type of storage material used. Typically, the temperatures required for desulfation are above 650° C. This is a critical parameter, since at low temperatures desulfation proceeds too slowly, and at excessively high temperatures the storage catalyst may be permanently damaged by heat.
German Laid Open Publication DE 198 27 195 A1 describes a method for desulfating a NOx storage catalyst arranged in the exhaust tract of a lean-burn internal combustion engine. To desulfate this NOx storage catalyst, the internal combustion engine is operated in several rich/lean cycles after a predefined desulfation temperature has been reached. In the rich phase of these cycles, the lambda value is reduced to preferably 0.95. This significantly accelerates the emission of the deposited sulfur as sulfur dioxide, while an undesirable hydrogen sulfide formation occurs only with a time delay. The duration of the rich phase is selected in such a way that no noticeable emission of hydrogen sulfide is ascertainable. The rich and lean periods of the cycles are preferably 2 to 10 seconds and 2 to 6 seconds, respectively.
German Laid Open Application DE 198 49 082 A1 describes a further method for desulfating a NOx storage catalyst arranged in the exhaust tract of a lean-burn internal combustion engine. After a predefined relatively low desulfation temperature of about 600° C. has been reached, the lambda value of the exhaust gas is initially reduced to a constant value of about 0.98 for a specific period of time. Subsequently, the internal combustion engine is operated with an oscillating lambda value, where the mean value is reduced from 0.98 to a value of 0.93 to 0.95 as a function of time. The oscillation frequency is between 0.1 and 0.2 Hz. To increase the desulfation rate, the mean catalyst temperature is increased to approximately 700 to 720° C.
Neither DE 198 27 195 A1 nor DE 198 49 082 A1 give any indication as to the means used to increase the temperature of the NOx storage catalyst to the desulfation temperature or as to how the temperature is measured. It is feasible, for example, to install a temperature sensor in the body of the NOx storage catalyst. Technically, however, this makes little sense because cracks starting from this installation site and propagating within the body of the NOx storage catalyst because of alternating thermal stresses during operation may eventually destroy the NOx storage catalyst.
German Laid Open Publication DE 100 26 762 A1 describes a further method for desulfating a NOx storage catalyst. Upstream and downstream of the NOx storage catalyst a thermocouple for measuring the temperature and a λ/NOx sensor are respectively disposed in the exhaust track. The system checks whether SOx regeneration is required by using a suitable mathematical model, a comparison of the lambda signals of a broadband sensor after the NOx storage catalyst during performance, a comparison of the lambda signal before and after the catalyst during NOx regeneration, or the signal of a NOx sensor during the lean phase. If it is determined that SOx regeneration is required, the exhaust temperature is first increased to the desulfation temperature of between 500 and 800° C. SOx regeneration is effected by alternating lean/rich operation. The intervals of the alternating lean/rich operation are optionally regulated by constant time periods or by temperature sensors in such a way that the exhaust temperature is maintained within a temperature range optimal for the desulfation process, i.e., between 500 and 800° C.
For the temperature increase during SOx regeneration, i.e., desulfation, DE 100 26 762 A1 proposes additional fuel injection (with and without torque component), late combustion, multistage combustion or external heating means. According to DE 100 26 762 A1, the temperature during desulfation is monitored either by a suitable mathematical model or by temperature sensors disposed upstream and downstream of the storage catalyst. Using a mathematical model to regulate the exhaust temperature appears too risky because this does not actually monitor the exhaust temperature so that damage to the catalyst as a result of excessively high exhaust gas temperatures cannot be excluded. Nor does measuring the temperature upstream of the catalyst provide any information on the actual temperature within the catalyst. Measuring the exhaust temperature downstream of the catalyst is also risky because an excessively high exhaust temperature is detected only after the catalyst has already been damaged.
The present invention addresses a short-coming of the prior art by providing a method for desulfating a storage catalyst that can be performed with little control complexity and that largely prevents damage to the storage catalyst due to excessively high temperatures during desulfation.